Many products we use every day are made from fibers. Examples of just a few of these products include paper, personal hygiene products, diapers, plates, containers, and packaging. Making products from wood fibers, cloth fibers and the like, involves breaking solid matter into fibrous matter. This also involves processing the fibrous matter into individual fibers that become fibrillated or frayed so they more tightly mesh with each other to form a finished fiber product that is desirably strong, tough, and resilient.
In fiber product manufacturing, refiners are devices used to process the fibrous matter, such as wood chips, pulp, fabric, and the like, into fibers and to further fibrillate existing fibers. The fibrous matter is transported in a liquid stock slurry to each refiner using a feed screw driven by a motor. Each refiner has at least one pair of circular ridged refiner discs that face each other. During refining, fibrous matter in the stock to be refined is introduced into a gap between the discs that usually is quite small. Relative rotation between the discs during operation causes the fibrous matter to be fibrillated as the stock passes radially outwardly between the discs.
One example of a refiner that is a disc refiner is shown and disclosed in U.S. Pat. No. 5,425,508. However, many different kinds of refiners are in use today. For example, there are counterrotating refiners, double disc or twin refiners, and conical disc refiners. Conical disc refiners are often referred to in the industry as CD refiners.
Each refiner has at least one motor coupled to a rotor carrying at least one of the refiner discs. During operation, the load on this motor can vary greatly over time depending on many parameters. For example, as the mass flow rate of the stock slurry being introduced into a refiner increases, the load on the motor increases. It is also known that the load on the motor will decrease as the flow rate of dilution water is increased.
During refiner operation, a great deal of heat is produced in the refining zone between each pair of opposed refiner discs. The refining zone typically gets so hot that steam is produced, which significantly reduces the amount of liquid in the refining zone. This reduction of liquid in the refining zone leads to increased friction between opposed refiner discs, which increases the load on the motor of the refiner. When it becomes necessary to decrease this friction, water is added to the refiner. The water that is added is typically referred to as dilution water.
One problem that has yet to be adequately solved is how to control refiner operation so that the finished fiber product has certain desired characteristics that do not vary greatly over time. For example, paper producers have found it very difficult to consistently control refiner operation from one hour to the next so that a batch of paper produced has consistent quality. As a result, it is not unusual for some paper produced to be scrapped and reprocessed or sold cheaply as job lot. Either way, these variations in quality are undesirable and costly.
Another related problem is how to control refiner operation to repeatedly obtain certain desired finished fiber product characteristics in different batches run at different times, such as different batches run on different days. This problem is not trivial as it is very desirable for paper producers be able to produce different batches of paper having nearly the same characteristics, such as tear strength, tensile strength, brightness, opacity and the like.
In the past, control systems and methods have been employed that attempt to automatically control refiner operation to solve at least some of these problems. One common control system used in paper mills and fiber processing plants throughout the world is a Distributed Control System (DCS). A DCS communicates with each refiner in the mill or fiber processing plant and often communicates with other fiber product processing equipment. A DCS monitors operation of each refiner in a particular fiber product processing plant by monitoring refiner parameters that typically include the main motor power, the dilution water flow rate, the hydraulic load, the feed screw speed, the refiner case pressure, the inlet pressure, and the refiner gap. In addition to monitoring refiner operation, the DCS also automatically controls refiner operation by attempting to hold the load of the motor of each refiner at a particular setpoint. In fact, many refiners have their own motor load setpoint. When the motor load of a particular refiner rises above its setpoint, the DCS adds more dilution water to the refiner to decrease friction. When the motor load decreases below the setpoint, dilution water is reduced or stopped.
During refiner operation, pulp quality and the load on the refiner motor vary, sometimes quite dramatically, over time. Although the aforementioned DCS control method attempts to account for these variations and prevent the aforementioned problems from occurring, its control method assumes that the mass flow of fibrous matter in the stock entering the refiner is constant because the speed of the feed screw supplying the stock is constant. Unfortunately, as a result, there are times when controlling the dilution water flow rate does not decrease or increase motor load in the desired manner. This disparity leads to changes in refining intensity and pulp quality because the specific energy inputted into refining the fibrous matter is not constant. These changes are undesirable because they ultimately lead to the aforementioned problems, as well as other problems.
Hence, while some refiner process control methods have proven beneficial in the past, they in no way have resulted in the type of control over finished fiber product parameters and the repeatability of these parameters that is desired. Thus, additional improvements in refiner process control are needed.